This invention relates to systems for receiving and/or transmitting electromagnetic energy and, more specifically, a covered inverted offset Cassegrainian antenna design.
Various antennas have been used to measure the atmosphere by way of electromagnetic radiation. Under such circumstances, rain, ice, and snow will degrade the performance of the antenna. That is, the measurements made from the electromagnetic waves using the antenna do not represent only the snow or rain in the atmosphere, but may develop a significant degree of dependence upon the degraded performance of the antenna resulting from rain, ice, or snow. Avoidance of this degradation is also important in millimeter wave communication and radar systems.
The following publications are helpful in explaining the background of the present invention:
(1) Blevis, B. C., "Losses due to rain on radomes and antenna reflecting surfaces," IEEE Trans. Antennas Propagation, vol. AP-13, pp. 175-176, January, 1965.
(2) Gibble, D., "Effects of rain on transmission performance of a satellite communication system," IEEE Intl. Convention Record, Part 6, p. 52, March, 1964.
(3) Hogg, D. C. and Ta-Shing Chu, "The role of rain in satellite communications," Proc. IEEE, vol. 63, no. 9, pp. 1308-1330, September, 1975.
(4) Anderson, I., "Measurements of 20 GHz transmission through a wet radome," IEEE Trans. Antennas Propagation, vol. AP-23, pp. 619-622, September, 1975.
(5) Giger, A. J., "4 Gc Transmission degradation due to rain at the Andover, Maine, Satellite Station," Bell Syst. Tech. Jour., vol. 44, no. 7, pp. 1528-1533, September, 1965.
(6) Jacobson, M. D., D. C. Hogg, and J. B. Snider, "Wet reflectors in millimeter-wave radiometry--experiment and theory," IEEE Trans. Geosc. and Remote Sensing, vol. GE-24, no. 5, pp. 784-791, September, 1986.
(7) Dragone, C. and D. C. Hogg, "The radiation pattern and impedance of offset and symmetrical near-field Cassegrainian and Gregorian antennas," IEEE Trans. Antennas Propagation, vol. AP-22, no. 3, pp. 472-475, May, 1974.
(8) Chu, T. S., R. W. Wilson, R. W. England, D. A. Gray, and W. E. Legg, "The Crawford Hill 7-meter millimeter-wave antenna," Bell Syst. Tech. Jour., vol. 57, no. 5, pp. 1257-1288, May 1978.
(9) Hogg, D. C. and R. A. Semplak, "An experimental study of near-field Cassegrainian antennas," Bell Syst. Tech. Jour., vol. 43, pp. 2677-2704, November, 1964.
In the discussion that follows, parenthetical references will be made to the above publications by number.
A technique which has been used to try to minimize weather degradation of an antenna has involved the use of a radome. However, both theory (1, 2, and 3) and experiment (4 and 5) show that the layer of liquid that forms on a radome during rain still has a tendency to degrade antenna performance, especially at millimeter wave lengths. Additionally, the system noise temperature where low-noise receivers are needed is also adversely affected. Wet reflectors do not appear to present as serious a problem with degradation (6, 1, and 3).
The bisected (or offset) Cassegrainian (or Gregorian) antenna is recognized to have several advantages (7). One advantage is that the aperture is not blocked such that the aperture efficiency and radiation patterns are very good. The impedance characteristics of such antennas are good with the feed horn being the only limitation in practice. The subreflector can be designed to be as large as desired without introducing blockage. Further, the cross-polarization discrimination is very good because of the long effective focal length.
Although this type of design has been implemented in antennas of very high quality that show superior performance in the above respects (8), such designs have included a significant undesirable feature. That undesirable feature is that the "standard" bisected (offset) Cassegrainian is highly susceptible to degradation caused by rain and wet snow falling at the site of the antenna.
It should be noted that the feed for a Cassegrainian antenna can be of various different types. For example, a near-field (plane-wave) type of feed uses a paraboloidal subreflector, thereby constituting a near-field Cassegrainian (9). Alternately, the feed might be an ellipsoidal reflector as discussed by Chu (8) to provide an appropriate beam waist with an hyperboloidal subreflector.
In addition to the above publications, the following patents (U.S. Patents unless indicated otherwise) are noted:
______________________________________ Pat. No. Inventor Date ______________________________________ 2,679,003 Dyke et al. May 18, 1954 2,679,004 Dyke et al. May 18, 1954 3,810,187 Hai May 7, 1974 3,850,504 Bisbee Nov. 26, 1974 3,995,275 Betsudan et al. Nov. 30, 1976 4,096,483 Bui Hai et al. Jun. 20, 1978 4,195,302 Leupelt Mar. 25, 1980 Japanese 51-92498 Mizusawa Feb. 1978 ______________________________________
The Dyke patents show a microwave antenna having a heater system and/or a snow detector and including drain holes incorporated in the center of a reflector portion of the antenna.
The Hai patent shows a Cassegrainian antenna having a cap which is metal and has an inner absorbent layer. The cap incorporates a screen to absorb diffracted waves and to prevent reflection of external parasitic waves and thus provide a better antenna pattern. Additionally, the cap is said to provide climatic protection of the antenna and is adapted to shift depending on the wind.
The Bisbee patent shows Cassegrainian telescope with an inflatable door to protect the telescope.
The Betsudan patent shows an antenna which is structured so that there is minimal effect from rain or snow on the primary radiator.
The Bui Hai patent shows a Cassegrainian antenna which is operable at two different frequency bands.
The Leupelt patent shows a Cassegrainian antenna whereby a thin foil is used to protect the horn from rain and snow.
The Japanese published patent application shows an antenna having a shield.
Although the above designs have been generally useful, they have often been subject to one or more of several significant disadvantages. For example, some of the designs do not allow the antenna to move. Other designs allow the antenna to move, but without maintaining a substantial degree of protection. Under some designs, the range of movement of the antenna is severely limited to avoid problems with water flow on the antenna surfaces some designs have required electromagnetic screens to provide good antenna patterns.